1. Field of the Invention
The present invention refers to an apparatus for the digital-analog conversion of a signal.
2. Description of the Related Art
Currently we are in the presence of a growing demand, above all in the market of radio communications, for high speed and high resolution digital-analog type converters (DAC), that is converters capable of converting a digital word, normally a word in code, into an analog signal.
An architecture that has been used to produce high speed and high resolution DACs includes an array of current sources. The digital-analog converter receives a digital input word that represents a desired output current and the current sources are selectively switched into an output to provide for the desired output current. Such types of converters have been used above all for driving resistive loads without the need for a voltage buffer.
One problem that troubles such converters lies in the decoupling of the current sources that diminishes the dynamic and static linearity of the converter, produces switching noise (presence of spikes of current in output) and decreases the output dynamics.
Several techniques exist to reduce the decoupling of the current sources in the DACs. One technique is described in D. Groeneveld al. “A Self-Calibration Technique for Monolithic High-Resolution D/A Converters”, IEEE Journal of Solid-State Circuits, vol. 24, pp. 1517–1522, December 1989; said technique comprises the use of an additional current source within a subarray that permits the extraction of a current source from the circuit every time for calibration and correction measuring purposes leaving a complete group of current sources available for the normal operations of digital-analog conversion.
The request of an additional current source can, however, degrade the dynamic performance of the converter since the input and output switchings of the current sources on the calibration ratio introduce spurious phenomena at the calibration frequencies in the output spectrum of the converter. In practice these spurious phenomena comprise additional dynamic components coming from dynamic decouplings among the arrays of current sources that are used every time. These dynamic decouplings are not attenuated by the calibration mechanism.